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Full text of "The Cambridge Natural History"

32

COMPOSITION   OF   STERNUM

brates, the sternum of the Mammalia consists of a series of pieces
as many as eight or nine or even sixteen in Ckoloepus, of whiel
the first is called the manubrium sterni, and the last the eiisiforiT,
cartilage, xiphisternum, or xiphoid process. The latter often
remains largely cartilaginous throughout life; in fact this ig
generally but not universally the case with that part of the
breastbone. The most extraordinary modification of the xiphoid
process is seen in the African species of the genus Manis, where
it diverges into two long cartilages, which run
back to the pelvis and then, curving round, run
forwards and fuse together in the middle line
anteriorly. These processes serve for the attach-
ment of certain tongue-muscles. They were looked
upon by Professor Parker as the equivalents of
the " abdominal ribs " of reptiles elsewhere non-
existent among mammals. This view is not,
however, usually held. The matiubrhmi sterni
is often keeled in the middle line below; this
is so with the Bats, which thus approach the
birds, and probably for the same reason, i.e. the
need of an enlarged origin for the pectoral muscle,
which is concerned in the movements of flight.
In many forms this part of the sternum is much
FIG. 19.—sternum broader than the pieces which follow; this is so

of the Pig (Sws        .  _    J.      -_..         ,    x     ^      .,     _..      .,              .

scrofa). x^. »w,  with the Viscacha.    In the Pig the precise reverse
Mesosteraum;  £s  seen  the manubrium being narrower than the

_2?#j  presterauni •

a», xipliisternum. rest of the sternal boiieleta It will be noticed,
OoteSo ylower's however, that in this and similar cases there are
no clavicles. Bibs are attached between the
successive pieces of the sternum. When the sternum is
reduced, as it is in the Cetacea and in the Sirenia, it is the
intermediate part of the series of bones which becomes abbrevi-
ated or vanishes. The Sperm "Whale has only a manubrium
sterni and a following piece belonging to the mesosternuni.
It is fair to say that the xiphoid process and the rest of
the sternum have disappeared, since among the Toothed Whales
a progressive shortening of the sternum can be seen. In
the Whalebone Whales the sternum is still further reduced ;
the manubrium is alone left, and to it are attached but
a single pair of ribs. In JSalaena, however, a rudimentary